Electrophoretic image display cells, hereinafter termed EPID cells, have been described in the art, for example in Evans, U.S. Pat. No. 3,612,758 and Ota, U.S. Pat. No. 3,668,106.
Generally, an EPID cell comprises firstly a dispersion of pigment particles in a dielectric liquid suspending medium to which is added a dye to cause the dispersion medium to have a contrasting color to that of the pigment particles and a charging material for introducing a charge on the pigment particles.
The dispersion is contained in a cell formed by two closely spaced electrodes joined together by a thin section of insulating material. One of the electrodes, the front or viewing electrode is transparent and the rear electrode, which is made to conform to the desired image or images desired to be displayed, may or may not be transparent.
An electric field is created across the suspension by means of a D.C. voltage applied to the front and rear electrodes.
Depending upon the charge on the pigment particles and the electrodes, the pigment particles are attracted to either the front or rear electrode.
If the pigment particles are negative, the front electrode is positive and the rear electrode is negative the pigment particles are attracted to the front electrode in the shape of the rear electrode. The observer sees an image of the color of the pigment against the color of the dispersion medium. Reversal of the polarity between the electrodes causes a reversal in the colors of the image and background.
Removal of the electric field does not cause disruption of the image, as the pigment particles remain in the previously activated position for a period of time. Thus the cell exhibits some memory function.
One problem with the known EPID cells is that they have relatively slow switching times, that is, the time it takes pigment particles to move from the front to the rear electrodes or the reverse. The relationship of the switching time to the applied voltage and distance between the electrodes is determined according to the formula EQU t = 6 .pi. d.sup.2 .eta. / V .epsilon. .zeta.
where t is the switching time, d is the distance between the front and rear electrodes, .epsilon. is the dielectric constant and .eta. is the viscosity of the suspending medium .zeta. is the zeta potential and V is the applied voltage.
As can be seen, the switching time is largely dependent on the distance between the electrodes as it increases with the square of this distance.
Due to practical considerations, there is a minimum limit on the distance between the electrodes and thus the minimum switching time for a given potential difference is limited for a particular suspension.
For many purposes a faster switching time then is achieved with the presently known EPID cells is highly desirable.
An additional problem is the need for use of dyes in order to hide the pigment when it is on the rear electrode.